The present invention relates to a biaxially-oriented polypropylene film having a high mechanical strength and to its use, especially as a base film for adhesive tapes. The film prepared in accordance with the present invention can be in the form of a monolayer or a multilayer film.
Adhesive tapes with base films of polypropylene are known in the art and have been described in the literature. However, the mechanical properties of the biaxially-oriented polypropylene films which are now conventionally used in this field do not meet the requirements for all applications of adhesive tapes.
Optimization of the mechanical properties of polypropylene base films, particularly an increase of the modulus of elasticity in the longitudinal direction, has always been the subject of intensive efforts, since these mechanical properties are closely related to the film's suitability for technical application and thus directly determine the processing characteristics.
Attempts to solve this problem, which have been described in the literature, comprise solutions based on process technology as well as modifications of the raw material, in particular by means of hydrocarbon resins.
One possibility of manufacturing high strength polypropylene films comprises a three-stage or multistage draw process such as described, for example, in EP-B-0 116 457. A manufacturing process of this kind has, however, the disadvantage that it requires additional equipment for the subsequent longitudinal drawing and is consequently very expensive. It is also highly susceptible to interruptions in the production sequence, for example, caused by film ruptures.
As compared with films which have only been biaxially drawn, these subsequently longitudinally drawn films moreover exhibit a markedly increased longitudinal shrinkage which, as a rule, makes it impossible for the films to withstand thermal drying --as is sometimes still employed after the application of adhesive compounds--without developing unwelcome shrink creases.
Modification of the raw materials used for the production of high-strength polypropylene films by means of various hydrocarbon resins is described, for example, in U.S. Pat. No. 3,937,762. Such a modification of raw materials makes it possible to produce polypropylene films, in which mechanical strength in the longitudinal direction may be significantly improved over films comprising unmodified raw materials but does not reach the values obtained with subsequently longitudinally drawn films and in which shrinkage in the longitudinal direction is also relatively high.
Excellent mechanical properties can be achieved by combining a resin addition to the raw material used with a subsequent longitudinal draw process. A corresponding procedure is described in EP-A-O 079 520. Moduli of elasticity in the longitudinal direction of 4,000 to 6,000 N/mm.sup.2 are attained. Nevertheless, even this process has the disadvantage of requiring a subsequent longitudinal draw process which is expensive and susceptible to faults.